There are basically two types of folding carts on the market today.
One type is represented by U.S. Pat. No. 4,411,343. It is essentially a device that opens into an "L" shaped body, 8" to 12" wide, with the longer part of the "L" being the handle by which the cart is pulled, and the shorter part of the "L" being the load platform. A wheel is placed approximately at each end of the right angle junction of the "L".
The advantage of this design is that it can be made compact, yet be able to accomodate large packages, since it has no sides. However, there are also several disadvantages.
Ideally, the best type of load for this cart, would be a small cube of uniform density, tht would sit at the junction of the "L", and balance on the cart, so that it is stable, and there is no tendency for the cart to rotate on its wheel axis.
As a practical matter, almost all loads are rectangular in shape, with one dimension being significantly much larger; such as suitcases, which are almost always not of uniform density. This requires that the load must be balanced on the 8" to 12" wide junction of the "L", so that the load doesn't slide off the cart.
Even if the load is perfectly balanced, any bump or hole in the ground, or any slight deviation of the ground from being flat and level, because of the small distance between wheels, can cause the cart to tilt, and the load to slide off, especially when making a turn with the cart.
If the load is tied to the cart, the whole cart may tip over.
Another problem with the fact that the load doesn't balance on the cart, is that the cart when being pulled is unstable. This requires that an up or down force must be applied to the handle of cart, while simultaneously pulling on the handle, depending on which side of the junction of the "L" has more weight.
On the market, there are heavy duty versions of folding carts of this type, which can be used for larger and heavier loads, which are more stable, and therefore easier to pull. However, this is accomplished by making the cart larger and heavier, and therefore less compact and portable; to hand carry.
Small wheels make the cart more compact when folded, and lower the center of gravity of the cart, making it more stable. However, small wheels have more friction than large wheels, and are more difficult to pull, especially with a heavy load, and when going up or down sidewalk curbs and stairs.
While larger wheels make the cart easier to pull, and to go up or down sidewalk curbs and stairs, and less effected by ground conditions; they also make for a higher center of gravity of the cart, making the cart easier to overturn, and less compact when folded.
In summing up this basic type of cart; it is small and compact, but its performance leaves much room for improvement.
The other basic type of folding cart on the market today, is the type used mostly for shopping. It is much larger than the first basic type previously mentioned, and consists, when opened, of a rectangular enclosure, with an open top and the rear of the enclosure extended upwards to form a handle. Two wheels are placed at each rear bottom end of the enclosure, and sometimes, two smaller wheels are added at the front bottom ends of the enclosure.
Typical dimensions of one of the larger type folding cart of this type are as follows: an enclosure of 15" front to back; 18" wide; and 22" high; with the extended rear of the enclosure 35" high to form a handle; use of 101/2" diameter rear wheels lifts the enclosure 6" off the ground, making the handle 41" high; the rear wheels are 22" apart, while the 6" diameter front wheels are 16" apart.
When folded, the cart is 45" high, 2" wide, except at the bottom where it is 15" wide because of the four wheels. The cart weighs about 22 lbs.
This type of folding cart, with 2 wheels, has a few performance advantages when compared to the first type of folding cart, previously mentioned. Having a wider distance between the larger wheels makes the cart more stable, and less effected by ground conditions. There is no problem with anything in the enclosure falling out of the cart, and it is easier to pull, although the problem with unbalanced loads still exists, and it is still necessary to either push down or lift up on the handle, while simultaneously pulling the cart.
This type of cart with four wheels, has all the above advantages of the two wheel model, plus a few more. Having four wheels, this cart can also be pushed as well as pulled. Pushing a four wheel cart is easier, since there is no problem with balancing the load.
The cart also is very much easier to use to go up or down sidewalk curbs or stairs, since the handle and front wheels can be used as a fulcrum to lift or lower the cart.
The main disadvantage of the four wheel cart, is that it is very difficult to maneuver with all four wheels on the ground, especially with a heavy load, since the wheels have a tendency to move best in a straight line, if perfectly aligned, which is very rarely the case. Usually, because of misalignment caused by uneven wear of the wheels, these carts tend to veer to one side, and the only way to change direction is to lift the front wheels of the cart off the ground and balance the cart on its rear wheels.
In summing up this second type of folding cart; it has many performance advantages over the first basic type previously mentioned, but because it is larger and heavier, it can hardly be considered as being compact, and use as a hand carried cart is impractical. A further limitation is that by using an enclosure, there is a size limitation imposed, and only objects that can fit in the enclosure can be carried, regardless if these objects do not weigh very much.
There is also a third basic type of folding cart on the market today, which enjoys very limited success because of its poor performance characteristics. This type consists of a non regid, cloth like enclosure, attached to a regid platform with wheels, at its bottom. However, this type is very difficult to use, since it must be pushed or pulled along by its top handles, or pulled by a strap, which makes it very difficult to go up or down sidewalk curbs, or steps, which can be best accomplished by lifting the whole cart. Further, the size of the load is limited by the size of the non regid enclosure.